1. Field of the Invention
The present invention relates to a sizing composition for glass threads, the said composition reacting under the action of heat. The present invention relates also to a process for the production of sized glass threads which are used, in particular, for reinforcing organic and/or inorganic materials, the said process using the composition mentioned above, and also to the glass threads obtained and the composites produced starting from the said threads.
2. Description of the Background
In the course of the text, "polymerisation", "polymerise", "polymerisable" are to be understood as meaning, respectively, "polymerisation and/or crosslinking", "polymerise and/or crosslink", "polymerisable and/or crosslinkable".
The manufacture of reinforcing glass threads is effected in known manner starting from streams of molten glass flowing from the orifices of one or more die plates. These streams are drawn mechanically in the form of continuous filaments and then the filaments are gathered into one or more basic threads which are subsequently collected on moving supports or undergo other treatments before being collected, depending on the desired applications.
Before they are gathered in the form of threads, the filaments are coated with a size by being passed over a continuously fed sizing member. The depositing of size is necessary in order to obtain threads and enables the threads to be associated with other materials, such as organic and/or inorganic materials, in order to produce composites.
The size acts primarily as a lubricant and protects the threads from the abrasion resulting from the high-speed friction of the threads on various members, such as guide members, during the above-mentioned process.
In the majority of cases, the size also ensures the integrity of the above-mentioned threads, that is to say, the interconnection of the filaments of which they are composed. This integrity, which is obtained especially after polymerising the size deposited on the threads, is generally desired in textile applications where the threads are subjected to great mechanical stresses. If the filaments constituting the threads are poorly integrated with one another they break more easily in the course of such applications and interfere with the operation of the textile machines. In general, the integrity of the threads is often desired because threads that do not exhibit this integrity are considered to be difficult to handle.
The size also provides the threads with the possibility of being associated with various materials to be reinforced with a view to the production of composite parts, by facilitating the wetting and/or impregnation of the threads with the materials to be reinforced and contributing to the creation of bonds between the said threads and materials. In particular, the mechanical properties of the composites obtained from the said material and threads depend on the quality of the adhesion of the material to the threads and on the suitability of the threads for being wetted and/or impregnated by the material.
The provision of suitable sizes for the processes used involves a search for sizing compositions that are sufficiently stable and compatible with the drawing speeds of the filaments that have to pass through them (several tens of metres per second). In particular, the sizes must be able to withstand the shearing induced by the passage of the filaments and must wet the surface of the filaments at the said speeds. Where the sizes used polymerise under the action of heat, it is also necessary for the sizes to have a sufficiently high reaction temperature to remain stable under the die plate. It is also desirable for such sizes to exhibit a substantial degree of conversion after polymerisation (this degree corresponding to the ratio between the number of functions that have reacted in the size after thermal treatment and the number of reactive functions in the size before thermal treatment) in order to ensure that sized glass threads having a constant quality are obtained (a size exhibiting a low degree of conversion being capable of changing over time) and in order to ensure a minimum loss of size in textile applications. Furthermore, in some cases, a high degree of conversion contributes to an improvement in the characteristics of the threads, such as the mechanical characteristics.
Added to these technical considerations are economic considerations and the ease with which the sizes can be used. For the latter reason in particular, the majority of sizes currently used are aqueous sizes which are easy to handle but which have to be deposited on the filaments in large quantities in order to be effective. These sizes comprise a substantial amount of water (90% by weight of the size, in particular for reasons of viscosity, which leads to sized threads containing from 6 to 12% by weight of water), which necessitates the provision of a stage for drying the threads before the threads can be used to reinforce organic and/or inorganic materials, since the water may impair the good adhesion between the threads and the materials. These drying operations are long and expensive, have to be adapted to the manufacturing conditions of the threads (the operations depend on the sizing composition used, on the mass of the threads to be dried . . . ) and their efficacy is not always optimum (especially with regard to rolls of threads or sheets of intermingled threads known as "mats"). In addition, when they are carried out during the fibre-forming operation (that is to say, before the threads obtained by gathering the filaments are collected), in respect of the filaments, as described in WO 92/05122 or in respect of the threads, as described in U.S. Pat. No. 3,853,605, they require the installation of drying devices under each die plate and when they are carried out on finished rolls of threads, they entail the risk of irregular and/or selective migration of the components of the size within the rolls (aqueous sizes already having a tendency, as soon as they have been deposited, to spread over the threads in an irregular manner, owing to their nature) to which are added, in certain cases, the phenomena of the threads' becoming coloured or the rolls' being deformed.
The deformation of the rolls is also observed in the absence of drying, on straight-edged rolls (rovings) of fine threads (that is to say, threads having a linear mass (titre) of the order of from 300 to 600 tex (g/km) or less) coated with aqueous sizes, the said rolls having a generally insufficient firmness (the firmness of such rolls being obtainable only in the case of large angles of intersection which are difficult to obtain in conventional processes for producing sized threads. The angle of intersection is to be understood as being the angle existing between the thread turns belonging to two consecutive layers and the bisecting line of which is contained in a plane perpendicular to the axis of the roll).
A small number of patents propose solutions for reducing the problems of drying sized filaments or threads by using non-aqueous sizes, but the sizes described in these patents generally use organic solvents which are tricky to handle and may impair the health of persons in the vicinity because they are toxic, and/or pose viscosity problems which are expediently solved by heating the said sizes, as described in U.S. Pat. No. 4,604,325 or by adding suitable agents as described in U.S. Pat. No. 4,609,591.
In many cases, the use of such sizes also requires that special devices be installed under each die plate; when the threads coated with these sizes are collected in the form of rolls, it is, in particular, necessary to treat the threads before obtaining finished rolls in order to prevent major adhesion between the turns of each roll, which would make it difficult to unwind the threads. These treatments, the effectiveness of which depends on the operating conditions, consist, for example, in polymerising the size on the threads by subjecting them, over at least a part of their path, to the action of ultraviolet rays in order to impart a sufficient integrity to them and to enable them to be handled as described in U.S. Pat. No. 5,049,407. In that case, however, the polymerised size prevents the filaments from sliding with respect to one another, this lack of mobility being prejudicial to the cutting of the threads (the threads bursting when cut owing to the mechanical degradation of the size) and possibly creating problems in textile applications where the threads used have to exhibit both integrity and pliability (the pliability of the sized threads being linked with the pliability of the size).
A readily handled thread coated with a purely organic size can be obtained without being subjected to a polymerisation treatment. Patent U.S. Pat. No. 4,530,860 describes such sizes which comprise a substantial percentage of film-forming agents, which then act as adhesive agents and impart an integrity to the threads, making them easier to handle.